The present invention relates to pesticides, and more particularly, to a method of providing a stable protective coating for UV sensitive pesticides.
The design of pesticides which do not accumulate in the environment has led to products with limited life and efficacy in the field due to solar UV sensitivity. These biopesticide materials consist of bacteria, nuclear polyhedrosis viruses, nematodes, and fungal spores. The short residual activity of biopesticides due to sunlight-induced UV degradation within hours of application decreases the usefulness and market impact that these products can have in selectively eliminating primary pests while maintaining beneficial secondary predator insects. Formulation of UV sensitive pesticides which do not have a rapid knockdown of pests prior to their degradation requires the addition of a UV protectant to extend their lifetime and efficacy.
Several methods have been tried to coat or encapsulate pesticides in order to protect them from adverse effects of the environment. While UV protection is a primary concern, control of the environment around biopesticides is also required since degradation from protein hydrolysis or activation of the pesticide is a function of pH and is further influenced by the presence of water, surfactants, and other additives which might be present during application of the agent. Furthermore, during the coating or protecting of biopesticides, process conditions must not degrade the pesticide by subjecting it to adverse solvents or reagents. The coating must not interfere with the bioavailability of the pesticide through either impalatibility or undigestibility. The coating must resist dissolving off of the biopesticide either during mixing or storage in a sprayable slurry formulation, or under rainfall conditions when applied as a powder.
Early methods of protecting pesticides utilized encapsulation techniques relying upon a two phase system consisting of an emulsified oil active and an aqueous polymer phase which would polymerize at the interface to create an insoluble coating. These methods, such as described in U.S. Pat. Nos. 4,056,610, 4,497,793, and 4,557,755, are multistep processes that require special equipment, use relatively expensive polymers and solvents, and are usually not amenable to biodegradable capsules. They are not useful in the preparation of biopesticide formulations due to the absence of an emulsion interface or lack of applicability to solid particles. Bohm et al (U.S. Pat. Nos. 4,948,586, and 4,844,896) extended this chemistry to biopesticides, but the method required multiple mixing and emulsifying steps; multiple reagents such as emulsifiers, glidants, crosslinkers, etc.; and organic solvents. Also, the product cannot be dried prior to application due to tackiness from residual high boiling solvents used in the preparation.
Improvements to the polymerization methods were made by using natural and biodegradable polymers. Lebo, et al (U.S. Pat. No. 5,552,149) demonstrates encapsulation by crosslinking a complex formed from lignosulfonates and proteins (such as high bloom gelatin). The benefit of UV protection from the lignosulfonate is demonstrated, and the disclosure extends to biopesticides. Moss in PCT/US92/03727 encapsulates Bt and other pesticides into a base of kraft lignin, polyethylene glycol (or acetone), and water by making a water-based emulsion in oil which is precipitated from the oil with acid. Although these two methods simplify previous encapsulations by eliminating complex polymerization and special equipment, the resulting products are not easily dried to give workable powders of well-controlled particle size when applied to biopesticides. Capsules made via the Moss method also contain solvent which may cause degradation of the biopesticide over time.
Polymerization of a gel matrix in the presence of a biopesticide leads to a solution of agents entrapped into a protective matrix. Shasha et al (U.S. Pat. No. 4,344,857) uses polyhydroxy xanthate copolymerization to generate a gel in the presence of a pesticide. This insoluble, pesticide-containing matrix is then filtered and granulated. Further drying leads to a friable solid which decomposes upon rewetting. If water stability is needed, crosslinking or hardening agents are required. The conditions for formation of the matrix are rather corrosive (H.sub.2 O.sub.2, FeCI.sub.3, FeSO.sub.4, or NaNO.sub.2 ; and strong acid to pH as low as pH 2.0) and reaction with the pesticide is not excluded. Spence et al (U.S. Pat. No. 4,223,007) uses sacrificial amounts of RNA or protein matter to form a matrix around biopesticides via precipitation of the protein into a gel, then breaking up the gel into microbeads, then crosslinking the gel for stability. Microbead wall thicknesses of 40 to 200 microns are used to increase microbial lifetime by 40%. Nelson et al (U.S. Pat. No. 4,753,799) utilizes alginates to form hydrogel capsules with the use of calcium chloride as a complexing or crosslinking agent which toughens the outer surface of the capsules. The 0.4 to 5 mm capsules are used in a slurry to preserve nematodes in a hydrated environment. Attractants can also be incorporated into the capsule. The slurry is sprayed into the environment and the capsule regulates the loss of water over time to extend the efficacy of the nematodes. Although these gel matrix procedures simplify the earlier encapsulation techniques, the resulting materials by nature do not provide materials that can be easily stored in a dry state and resuspended at a later time.
Dimitri (U.S. Pat. No. 3,929,453 and Re. 29,238) simplifies the process of entrapping biological actives into a matrix by forming coprecipitation-inclusion composites from a mixture of kraft lignin and the pesticide. The lignin provides a protective coating and acts as a UV protectant against sunlight induced degradation. The composites are rain fast and inhibit the action of microorganisms, yet regulate the release of the active either by diffusion through the lignin matrix or by degradation or alkali dissolution of the matrix. A solution of alkali lignin salt is mixed with the biological active and emulsified. The lignin is then solidified by coprecipitation-inclusion from the alkaline solution by acidification. The product is then isolated from the co-solvent and the remaining water removed by evaporation. The matrices obtained are solids varying in dimension from 0.5 to droplet size with active to lignin ratios of 0.1:1 to 10:1.
Lebo (U.S. Pat. No. 5,529,772) improves upon the Dimitri and Spense inventions in the case of protecting biopesticides by utilizing the ability of lignosulfonates to complex with proteins. The active protein toxin in the biopesticide is reacted with the lignosulfonate to form a stable complex having the UV protecting lignin as an integral part of its structure. This is done by mixing the lignosulfonate with the biopesticide and acidifying the mixture to below the isoelectric point of the complex where the complex becomes an insoluble precipitate. The precipitate can be used as a slurry or can be isolated and dried to a powder. This procedure also improves upon the use of lignin as an adjuvant as in Hobbs (PTO/US95/01760). The UV protectant is integrally incorporated on the surface of the pesticide where it is most effective, rather than simply added to a pesticide formulation where dilution can decrease the effectiveness of the UV protectant. The pesticide is then released from the lignin in the caustic insect gut upon ingestion of the complex by the host insect.
The technology for providing UV protection to biopesticides has moved from the complex microencapsulation techniques used for chemical pesticides to the simpler coacervation methodologies, yet these methods are not prevalent in the market today. Accordingly, there remains a need for an improved process for providing UV protection to pesticides.